1. Field of the Invention
This invention relates to a method of processing a radiographic image used for medical diagnosis and an apparatus therefor, and more particularly to a method of and apparatus for processing a radiographic image when the radiographic image once recorded on a radiographic film is copied on a recording medium.
The radiographic image referred to in this application means an original X-ray image recorded on an X-ray film by the conventional radiographic method. The present invention is applied to the radiographic image copying system in which the radiographic image is once converted to an electric signal and the electric signal is used for reproducing the radiographic image on a final recording medium like an ordinary photographic film. When the electric signal is used for reproducing the radiographic image, the electric signal is processed to obtain an improved image having high diagnostic efficiency and accuracy.
2. Description of the Prior Art
Since X-rays do harm to the human body, it is impossible from the viewpoint of safety to expose the human body to X-rays of high dose. Therefore, it is desirable that the necessary information in the radiography can be obtained by exposing the human body only once to the X-ray of comparatively low dose. On the other hand, radiographs should preferably have both a wide exposure latitude and high image quality of high contrast, high sharpness, low noise etc. for viewing and diagnosis. Unfortunately, since the conventional radiography is designed to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information and the image quality are both insufficient and neither of these properties are completely satisfied.
In view of the above defects in the conventional radiography, the inventors of the present invention have proposed a radiographic image processing method in which the radiographic image is read out by an electronic image read-out device to convert the radiographic image into an electric signal and the electric signal is processed to enhance the diagnostic efficiency and accuracy of the image when the image is reproduced on a recording medium like a photographic film by improving the sharpness and the granularity of the image, as disclosed in Japanese unexamined Patent Publication No. 54(1979)-121043. This method remarkably improve the diagnostic efficiency and accuracy as compared with the conventional radiography.
Further, the inventors of the present invention have found a new process and apparatus which solve the above mentioned problems and satisfy both the wide exposure latitude and the high image quality of high contrast, high sharpness and low noise in the radiographic image copying system as disclosed in a copending application; U.S. patent application Ser. No. 106,734. According to this method and apparatus, a radiographic image having high diagnostic efficiency and accuracy can be economically obtained at a high speed.
The method and apparatus of the above mentioned copending application are characterized in that, in the course of scanning a radiographic film to read out the radiographic image information recorded thereon, converting the read out information into an electric signal and recording a visible image on a recording medium using the electric signal, an unsharp mask density Dus corresponding to the super-low frequency is obtained for each scanning point and a signal conversion represented by a formula EQU D'=Dorg+.beta.(Dorg-Dus)
where Dorg is an original image density read out from the radiographic film and .beta. is an emphasis coefficient is performed to emphasize the frequency component above the super-low frequency. In this method and apparatus, the emphasis coefficient .beta. may be a fixed value or a variable value. In the latter case, the emphasis coefficient .beta. may be varied with either Dorg or Dus.
However, according to the research and investigations conducted further thereafter by the inventors of the present invention, the artifact is liable to appear in the low and high density regions when the emphasis coefficient .beta. is fixed. When the emphasis coefficient .beta. is made variable, on the other hand, it was found that, in case that .beta. was monotone increasing function (.beta.'.gtoreq.0) for instance, the artifact was prevented from appearing in the low density region where Dorg or Dus was small, but it was difficult to prevent the appearance of the artifact in the form of black lines in the muscle area around the boundary of a muscle and a bone for example. In other words, in the above described method, it was difficult to completely prevent from occurring artifacts such as the low density portions around the boundaries of edges in an image of bones and muscles becoming saturated white to the fog level of the recording medium or the high density portions around the boundaries becoming black lines, or the contour lines appearing around the contour of the barium sulfate contrast medium over-emphasized in a radiographic image of a stomach (Magen). Thus, in the above described method it was difficult to sufficiently improve the diagnostic efficiency and accuracy of the finally obtained radiographic image and there was a fear of making an error in diagnosis in some cases due to the artifacts.